This invention relates to electrical switches and in particular, to a multi-element switch matrix constructed to provide a rapid scanning of its elements.
A membrane switch switches electrical current across two contacting electrodes, at least one of which is attached to the rear surface of a flexible membrane. Pressing the front surface of the membrane near the contact deforms the membrane to bring the contact into electrical communication with the second contact opposing the first. The second contact may be attached to the front surface of a second flexible membrane rigidly supported by another surface.
The two membranes are typically spaced from each other by small insulating spacers distributed across the area between the membranes. The spacers permit the opposing contacts to touch only when the front membrane is pressed.
The contacts may be printed to the membranes by using conductive inks or deposited by vacuum metalization techniques or other methods known in the art. The conductors attaching the contacts to other circuitry may also in part be printed or deposited on the membranes.
Membrane switches find wide spread use in applications where sealed or protected switches or operating panels are needed. The front membrane provides a natural sealing of the contacts from outside contaminants.
Because the membranes and electrodes may be made transparent, one important application of membrane switches is for touch panels associated with CRT, LCD, Plasma, Electroluminescent displays. Here, a display directs the user to particular locations on the transparent membrane switch where pressing will activate a membrane switch. The number and caption of the switches may be readily changed (by changing the image on the display) thus providing a keyboard flexibly adapted for use with hierarchical menu structures in which a decision to press one key produces an entirely new keyboard with new choices.
It is often desired that a single pair of membranes support a number of separately operable contacts. These contacts may be arranged in a matrix of rows and columns on the membranes. In order to reduce wiring and circuitry for the matrix of contacts, each contact pair is normally scanned by row and column. For example, one terminal of the switch formed by each pair of contacts in each row may be connected to a common row line, and the other terminal may be connected to a common column line. These shared lines substantially reduce the amount of wiring to the switches.
In order to detect the closure of a switch, a unique voltage is applied to one row at a time and voltages at the columns are monitored to see which switches have closed. This process is repeated for each row, one at a time, on a rapid basis to ensure that even the briefest expected closure of a switch will be detected.
When the unique voltage is applied to a particular row, the other rows are connected to a different voltage by high impedance to prevent excessive currents in the event that more than one switch in a column is depressed. In a typical circuit, the rows may be driven low and the columns are passively pulled high to a voltage level above the switching threshold of the monitoring circuitry.
The pull up resistors must be large compared to the intrinsic resistance in the conductors and the expected contact resistance of the switches in order for proper detection threshold to be reached when switches are depressed. High resistance pull-up resistors are especially important in touch panels with transparent conductors which have high intrinsic resistances. The pull up resistors must also be large in case many switches in the array are depressed at once, which effectively places the pull up resistors in parallel. During this scanning process, the voltage on successive rows of the switches must be changed rapidly. The intrinsic resistance of the conductors to the switch contacts and the capacitance between the contacts (and added capacitance for the suppression of electromagnetic interference EMI) severely limits the speed with which such voltage changes may be accomplished. This, in turn, places practical limits on the number of switches that may be scanned within the prescribed time limits.